1. Field of the Invention
The present invention relates to a negative-working lithographic printing plate precursor comprising a novel polymerisable compound.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
The so-called “analogue” printing plates are generally obtained by first applying a so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an imagesetter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called “computer-to-plate” (CtP) method has gained a lot of interest. This method, also called “direct-to-plate”, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a platesetter. A printing plate precursor for CtP is often called a digital plate.
The support of the lithographic printing plates are typically aluminum supports which have a hydrophilic surface or on which a hydrophilic layer has been provided. This hydrophilic surface and/or layer should improve the water acceptance of the non-printing areas of a lithographic printing plate and the repulsion of the printing ink in these areas. During developing the soluble portions of the coating should be easily removed whereby the surface of the support remains residue-free so that clean background areas are obtained during printing.
Digital plates can roughly be divided in three categories: (i) silver plates, working according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates containing a photopolymerisable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion.
Photopolymer printing plates rely on a working-mechanism whereby the coating—which typically includes free radically polymerisable compounds—hardens upon exposure. “Hardens” means that the coating becomes insoluble or non-dispersible in the developing solution and may be achieved through polymerization and/or crosslinking of the photosensitive coating upon exposure to light. Photopolymer plate precursors can be sensitized to blue, green or red light i.e. wavelengths ranging between 450 and 750 nm, to violet light i.e. wavelengths ranging between 350 and 450 nm or to infrared light i.e. wavelengths ranging between 750 and 1500 nm. Optionally, the exposure step is followed by a heating step to enhance or to speed-up the polymerization and/or crosslinking reaction. The presslife of photopolymer plates is related to the cohesive strength within the polymerized photolayer. The higher the cohesive strength, the higher the presslife. The cohesive strength can preferably be improved by increasing the crosslinking degree and/or by supramolecular non-covalent interactions such as H-bonding, Van der Waals interaction and dipole-dipole interactions.
In general, a toplayer or protective overcoat layer over the imageable layer is required to act as an oxygen barrier to provide the desired sensitivity to the plate. A toplayer typically includes water-soluble or water-swellable polymers such as for example polyvinylalcohol. Besides acting as barrier for oxygen, the toplayer should best be easily removable during processing and be sufficiently transparent for actinic radiation, e.g. from 300 to 450 nm or from 450 to 750 nm or from 750 to 1500 nm.
The classical work flow of photopolymer plates involves first an exposure step of the photopolymer printing plate precursor in a violet or infrared platesetter, followed by an optional pre-heat step, a wash step of the protective overcoat layer, an alkaline developing step, and a rinse and gum step. Over the past years, there is a clear evolution in the direction of a simplified workflow where the pre-heat step and/or wash step are eliminated and where the processing and gumming step are carried out in one single step. However, it is known that the developing step becomes much more critical when the alkaline developer is replaced by a gum solution with a lower pH. For example, the hydrophilic polymers in the protective overcoat layer result in a problematic viscosity increase of the gum solution limiting the exhaustion behavior of the plate. Therefore, besides the need to reduce material cost or to avoid the investment for additional coating equipment, there is a high interest in the industry to eliminate the protective overcoat layer present on photopolymer plates. The elimination of the toplayer however negatively impacts the sensitivity and/or presslife of the printing plate.
EP 919 868 discloses a positive type image forming material comprising a polymer (A) containing a phenolic hydroxyl group which is soluble in an aqueous alkaline solution and a light and heat decomposing compound (B) which suppresses the solubility of polymer (A) and a cross-linkable compound (C) which increases the solubility-suppressing effect of compound (B).
U.S. Pat. No. 4,782,005 discloses a radiation sensitive composition comprising an acrylic or methacrylic acid ester, a photoinitiator, and a small amount of the additive N,N′-diorgano dicarboxamide. The radiation sensitive composition is sensitive to ultraviolet light and the N,N′-diorgano dicarboxamide is present in the composition in a minor amount ranging between 1 and 10% wt.
U.S. Pat. No. 7,279,255 discloses a thermal photopolymer plate without a toplayer nor need for a preheat step which has a good shelf life because of the incorporation of a specified poly(alkylene glycol) primary additive into the coating of the plate.
US 2009/0186299 describes a single-layer thermal photopolymer plate where the imageable element includes an organic adhesion promoter having an ethylenically unsaturated carbon-carbon double bond connected to an alkoxysilyl or hydroxysilyl group.
U.S. Pat. No. 8,329,383 discloses a light sensitive printing plate which require no protective overcoat layer nor a preheat step and which includes carboxylamino compounds as oxygen scavenger and shelf life stabilizer.
US 2009/0142702 discloses a method for making a light sensitive, negative-working photopolymer printing plate which is gum processable by including in the coating a specific compound including two hydroxyl groups.
Although many attempts have been made in the art to provide single-layer photopolymer printing plates without any loss in sensitivity and plate performance, there is still an urgent need to improve the lithographic properties such as for example sensitivity and presslife of such overcoat-free printing plates.